For high density polyethylene, hereinafter referred to as polyethylene, the molecular weight distribution (MWD) is one of the basic properties that determines the properties of the polymer, and thus its end-uses.
Although it may be difficult to evaluate the influence of each property taken independently, it is generally accepted that the molecular weight mostly determines the mechanical properties while the molecular weight dispersion mostly determines the rheological properties.
There is a demand for high molecular weight polyethylene, because an increase of the molecular weight normally improves the physical properties of the resins. However, high molecular weights tend to make polymers harder to process. On the other hand, an increase in the MWD tends to improve the flowability at high shear rate during the processing. Thus, broadening the MWD is one way to improve the processing of high molecular weight (=low melt flow index) polyethylene, in applications requiring fast processing at fairly high die swell, such as in blowing and extrusion techniques.
Some believe that, in polyethylene having a high molecular weight combined with a broad MWD, the lower molecular weight portion aids in processing while the higher molecular weight portion contributes to the good impact resistance of the film, such polyethylene being processed at higher throughput rates with lower energy requirements.
The MWD may be described completely by the curve obtained by gel permeation chromatography. However, the MWD is generally described by a figure which is a good evaluation, also called the polydispersity index, representing the ratio of the weight average to the number average molecular weight.
Depending on the applications, the required MWD will range from 10 to 30.
It was first proposed to prepare polyethylene having broad MWD by blending polyethylenes having different molecular weights in order to obtain the advantages of a broad distribution. However, the results were not satisfactory as a blend does not behave like an intimate mixture of polyethylenes prepared in situ.
It has also been proposed to use two-step reactions in one reactor. Examples of such processes are described in GB-1174542-A, GB-2020672-A and BE-883687-A.
It has further been proposed to use several reactors connected in series.
For this purpose a process is known for the preparation of polymodal ethylene polymer in which ethylene is polymerized in two stages in the presence of a halogen-containing organoaluminum compound, a transition metal compound and different amounts of hydrogen in each stage (GB 1233599).
A process is furthermore known for the preparation of olefin polymers by a two-stage polymerization, a high molecular polymer being prepared in the first stage at a low H.sub.2 /C.sub.2 H.sub.4 ratio and a low-molecular polymer being prepared in the second stage at high H.sub.2 /C.sub.2 H.sub.4 ratio (EP-A 57,352). The catalyst used is, inter alia, a halogen-containing organoaluminum compound together with the reaction product of an oxygen-containing organomagnesium compound and an oxygen-containing organotitanium compound, both of which are in solution, and an aluminum halide. A similar process is disclosed in EP-57420-A.
It has also been proposed a process to polymerize ethylene in two stages according to which the pressure in the second reactor is kept lower than in the first one; the polymerization is carried out in the presence of usual Ziegler-Natta catalyst such as a transition metal catalyst supported on a solid carrier and an organoaluminum compound. Examples of such processes are described in U.S. Pat. Nos. 4,414,369 and 4,338,424.
However, the ethylene polymers obtained with such processes are not very convenient as regard to their mechanical properties. It has now been found that the prior art processes involving two liquid-full loop reactors connected in series could be improved.
It is therefore an object of the present invention to provide an improvement to such processes for the copolymerization of ethylene to form ethylene copolymers with good processability, good physical properties and diverse applicability.
It has been found that this object can be achieved with a two-stage process involving liquid full loop reactors connected in series, the improvement consisting of using one or more settling legs of the first reactor for the transfer.